1. Field of the Invention
The present invention relates to a wireless tag reader/writer apparatus. Particularly, the invention relates to a wireless tag reader/writer apparatus which can receive data from, and transmit data to wireless tags from a batch of articles a method of detecting dropping-out of wireless tags, and a wireless tag.
2. Description of the Related Art
Wireless tags (RFID: Radio Frequency Identification) according to this invention are chip-shaped responders, each having at least a memory and an antenna and being able to communicate (receive and transmit data) with a wireless tag reader/writer apparatus (hereinafter called “reader/writer apparatus”) via the antenna connected to an interrogator.
In recent years, wireless tag systems have been used in rapidly increasing numbers, in which wireless tags are attached to articles and a reader/writer apparatus reads data from the wireless tags.
If a wireless tag is attached to, for example, a bundle of envelopes and stores data representing the number of envelopes, how many envelops constitute the bundle can be readily known by reading the data from the wireless tag, without necessity of counting the envelopes. This is desirable in view of the work efficiency. However, the reader/writer apparatus may fail to read the data from the wireless tag, depending on the condition of the wireless tag. In this case, the envelopes must be counted in order to determine whether all data has been read from the wireless tag. This not only requires the use of a counter, but also reduces the working efficiency.
To solve this problem, Jpn. Pat. Appln. Laid-Open Publication No. 2006-185391 discloses a reader/writer system. In this system, not only the ID data of an RF tag, but also the related data about the RF tags preceding and following the RF tag is recorded in the RF tag. From the related data, it is determined whether any envelope attached with an RF tag has slipped from the bundle.
Jpn. Pat. Appln. Laid-Open Publication No. 2007-33259 discloses a wireless tag data reading apparatus. This apparatus stores the data items read from two adjacent wireless tags, and reliably determines that any wireless tag has been moved from the article or has failed to work.
In the technique disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2006-185391, however, the data of only one or two tags, each preceding or following the tag attached to an article, is stored as related data. Hence, when two or more articles, such as envelopes, dropout, or the wireless tags drop from these articles, this fact cannot be detected.
The technique disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2007-33259 detects the order in which articles are arranged, each having a wireless tag attached to it, or detects the dropping-out of any article from the others. The articles of one batch must be arranged in exactly the same order as before. This inevitably decreases the efficiency of inspecting the articles.